The present invention relates to a system for controlling an electric induction motor and, more particularly, to rotor flux observer design for controlling an electric induction motor.
Typical induction motors include a rotor mounted inside a stator for rotation about a rotation axis. A common rotor design includes a xe2x80x9csquirrel cage windingxe2x80x9d in which axial conductive bars are connected at either end by shorting rings to form a generally cylindrical structure. The stator is formed by a plurality of winding which surround the rotor and are typically arranged in three separate phases. Time varying voltage is applied across the stator windings which generate an electromotive force (emf) and an associated stator magnetic field which rotates around the stator at a stator field frequency primarily within the space defined by the rotor.
As the stator field rotates about the rotor, relative motion between the stator field flux and the rotor bars induces voltages in the rotor at a slip frequency. Slip is the difference between the stator frequency. Slip is the difference between the stator field frequency and the rotor electrical speed. The voltages induced in the rotor cause rotor bar currents, which in turn generate a rotor magnetic field. The stator and rotor fields are stationary with respect to each other, but are separated by a changeable rotor angle (a). The two fields interact to produce torque, which causes rotor rotation. The amount of motor torque for an induction motor will vary as the voltages induced in the rotor change.
It is known that for efficient control of an induction motor, the rotor position must be measured or estimated. A rotor position sensor is typically employed to sense the position of the rotor and feed it directly to a controller or other device that is controlling the operation of the motor. Based on a reading from the position sensor, in field oriented control of an induction motor, the orientation of rotor flux is determined and the stator voltage signal is calculated. While position sensors have been widely used for sensing the rotor position, they can be unreliable, are relatively noisy, and are relatively expensive.
Various control schemes for torque control of induction motors (IM) without the use of a position sensor have been employed. These IM torque control schemes include as a part the flux estimation procedure, the rotor flux observer, which is based on the following known rotor flux dynamic equation:                                           ⅆ                                          λ                ^                            r                                            ⅆ            t                          =                                            (                                                                    -                                                                  R                        r                                                                    L                        r                                                                              ⁢                  I                                +                                                      ω                    r                                    ⁢                  J                                            )                        ⁢                                          λ                ^                            r                                +                                                    R                r                                            L                r                                      ⁢            M            ⁢                          xe2x80x83                        ⁢                          i              s                                                          (        1        )                                                      Where            ⁢                          xe2x80x83                        ⁢                                          λ                ^                            r                                =                      [                                                            λ                  ^                                rx                                                              λ                  ^                                ry                                      ]                          ,                              i            s                    =                      [                                          i                xs                                            i                ys                                      ]                          ,                  I          =                      [                                                            1                                                  0                                                                              0                                                  1                                                      ]                          ,                  J          =                      [                                                            0                                                                      -                    1                                                                                                1                                                  0                                                      ]                                              (        2        )            
In the equations above, {circumflex over (xcex)}r is the rotor flux estimation, is is the stator current, and xcfx89r is the rotor electrical speed and Rr, Lr, and M are the rotor resistance, rotor inductance, and machine mutual inductance, respectively.
For higher rotor speeds, the discrete implementation of equation (1), shown below:                                           λ            ^                                rk            +            1                          =                                            λ              ^                        rk                    +                      T            ⁡                          [                                                                    (                                                                                            -                                                                                    R                              r                                                                                      L                              r                                                                                                      ⁢                        I                                            +                                                                        ω                          rk                                                ⁢                        J                                                              )                                    ⁢                                                            λ                      ^                                        rk                                                  +                                                                            R                      r                                                              L                      r                                                        ⁢                  M                  ⁢                                      xe2x80x83                                    ⁢                                      i                    sk                                                              ]                                                          (        3        )            
where T is a sampling period, can be numerically unstable. The solution of (1) is oscillatory with frequency xcfx89r, and the simple Euler integration method implemented in equation (3) is not adequate for high rotor speeds. Thus, an improved solution for controlling the motor torque that operates reliably at all speeds would be advantageous.
It is an object of the present invention to provide a discrete time flux observer for torque control of induction motors.
It is another object of the present invention to provide a flux observer that provides significantly improved performance at high rotor speeds.
It is yet another object of the present invention to provide a flux observer that provides an accurate flux estimation that provides increased endurance and ease of maintenance preserving high dynamic performance.
In accordance with the above and other objects of the present invention, a system for controlling the torque of an induction motor is provided. The system includes a controller that receives a signal which is representative of the motor stator current. The controller also receives information on the rotor speed. The actual rotor speed or the estimated rotor speed is then fed to a flux observer. The flux observer calculates the rotor flux which is necessary to calculate the stator voltage to control the output torque of the induction motor. The observer is a discrete time flux observer that utilizes analytical solutions of the rotor flux dynamic equation over a time sampling interval and thus provides higher accuracy and improved performance over standard Euler or Tustin approximations.
These and other features of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanied drawings and appended claims.